In recent years, attention has been directed to techniques for converting natural energy such as solar light and wind power into electrical energy. In addition, there has been an increasing demand for a non-aqueous electrolyte secondary battery as a high energy-density battery capable of storing a large amount of electrical energy. Among non-aqueous electrolyte secondary batteries, a lithium ion secondary battery is promising because of their light weight and high electromotive force. However, it is difficult to use continuously a lithium ion secondary battery at a high temperature of, for example, 80° C. or more. Moreover, the cost of a lithium resource is increasing. Thus, development efforts have been directed at a sodium ion secondary battery which can be stably charged and discharged even in a high temperature environment, as a heat resistant battery. For example, a temperature in usage environment for a sensor provided in a high temperature furnace (electric furnace) ranges from 120 to 300° C. Therefore a battery used as a power source of such a sensor needs to be heat resistant equal to that of the sensor.
In a sodium ion secondary battery, a sodium ion-conductive electrolyte is used. The electrolyte is also required to be heat resistant considering a high temperature environment in which a battery is used. Thus, as a electrolyte having high heat resistance and a low melting point, a mixture of sodium bis(fluorosulfonyl)imide (NaFSA) and potassium bis(fluorosulfonyl)imide (KFSA) has been developed. Moreover, there has been proposed a salt of an organic cation and a sulfonyl anion, a salt of an organic cation and a fluoroborate anion, and the like (Patent Literature 1).